slightly better deprecation message for fn syntax

[Gankra]

Came up on IRC that this was a bit unhelpful as to what should actually be done. I am new to changing compiler messages, please let me know if there's anything else that needs to be done to accomadate this change.

(My build system is still constantly crashing [Is bors contagious?], so this hasn't been formally checked. I figure it's a simple enough change that any consequences [like compile-fail expected messages?] can be eyeballed by someone more experienced.)

[ftxqxd]

Nit: this comma probably needs a space after it.

(Also, maybe the other comma (on the line above) should be changed to a semicolon?)

[Gankra]

semicolon sounds good. I would probably just kill the comma after e.g. upon reflection.

[Gankra]

@P1start cleaned the message up

[alexcrichton]

Are closures also covered by this warning? For example:

type A = <'a>|&'a int| -> &'a int;

Just wanna make sure we're not recommending fn syntax for a closure.

[Gankra]

I honestly have no idea. It seems to be the only deprecation notice for using the old syntax, but I'm unclear as to the scope that new syntax applies to.

[alexcrichton]

All I'm really getting at is that this same message is used for one of two locations for the deprecated syntax, and the suggested fix only applies to one of the locations:

type A = <'a> ||;
type B = fn<'a>();

$ rustc foo.rs
foo.rs:1:11: 1:13 warning: deprecated syntax, use `for` keyword now
foo.rs:1 type A = <'a> ||;
                   ^~
foo.rs:2:13: 2:15 warning: deprecated syntax, use `for` keyword now
foo.rs:2 type B = fn<'a>();
                     ^~

[Gankra]

The form of the fix is that same, right? <'a>|| -> for<'a>||? It is an ``e.g.`, so it doesn't strictly claim to be exhaustive. I can update it to be exhaustive, if that's what you're looking for.

[alexcrichton]

hm ok I guess this is ok for now with the "e.g." in front, sorry for the confusion!